JLink、JTAG接口详细图解「建议收藏」

大家好，又见面了，我是你们的朋友全栈君。

说明：

1脚：通常连接到目标板的vdd，用来检测目标系统是否供电；检测原理上图中有简单的说明。

2脚：原版的JLink这个引脚没有使用，不提供Vsupply输出，而很多改造版的JLink通过跳线选择从该引脚输出3.3V的电压给外边，我的就是这样的。

可以到网上找JLink的原理图看看。

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0517b/Cjaeccji.html

JTAG interface signals

The following table describes the signals on the JTAG interfaces:

Table 1. JTAG signals

Signal	I/O	Description
DBGACK	–	This pin is connected in the RVI run control unit, but is not supported in the current release of the software. It is reserved for compatibility with other equipment to be used as a debug acknowledge signal from the target system. It is recommended that this signal is pulled LOW on the target.
DBGRQ	–	This pin is connected in the RVI run control unit, but is not supported in the current release of the software. It is reserved for compatibility with other equipment to be used as a debug request signal to the target system. The RVI software maintains this signal as LOW. When applicable,RVI uses the scan chain 2 of the processor to put the processor in debug state. It is recommended that this signal is pulled LOW on the target.
GND	–	Ground.
nSRST	Input/output	Active Low output from RVI to the target system reset, with a 4.7kΩ pull-up resistor for de-asserted state. This is also an input to RVI so that a reset initiated on the target can be reported to the debugger. This pin must be pulled HIGH on the target to avoid unintentional resets when there is no connection.
nTRST	Output	Active Low output from RVI to the Reset signal on the target JTAG port, driven to the VTref voltage for de-asserted state. This pin must be pulled HIGH on the target to avoid unintentional resets when there is no connection.
RTCK	Input	Return Test Clock signal from the target JTAG port to RVI. Some targets must synchronize the JTAG inputs to internal clocks. To assist in meeting this requirement, you can use a returned, and retimed, TCK to dynamically control the TCK rate. RVI provides Adaptive Clock Timing, that waits for TCK changes to be echoed correctly before making more changes. Targets that do not have to process TCK can ground this pin. RTCK is not supported in Serial Wire Debug (SWD) mode.
TCK	Output	Test Clock signal from RVI to the target JTAG port. It is recommended that this pin is pulled LOW on the target.
TDI	Output	Test Data In signal from RVI to the target JTAG port. It is recommended that this pin is pulled HIGH on the target.
TDO	Input	Test Data Out from the target JTAG port to RVI. It is recommended that this pin is pulled HIGH on the target.
TMS	Output	Test Mode signal from RVI to the target JTAG port. This pin must be pulled HIGH on the target so that the effect of any spurious TCKs when there is no connection is benign.
Vsupply	Input	This pin is not connected in the RVI unit. It is reserved for compatibility with other equipment to be used as a power feed from the target system.
VTref	Input	This is the target reference voltage. It indicates that the target has power, and It must be at least 0.628V. VTref is normally fed from V_dd on the target hardware and might have a series resistor (though this is not recommended). There is a 10kΩ pull-down resistor on VTref in RVI.

ARM系统的JTAG接口的设计不当往往使硬件系统无法调试，所以在设计ARM系统前要先熟悉ARM系统的JTAG接口的定义和常见问题。

1．ARM系统的JTAG接口是如何定义的？每个PIN又是如何连接的？

下图是JTAG接口的信号排列示意：

接口是一个20脚的IDC插座。下表给出了具体的信号说明：

表 1 JTAG引脚说明

序号	信号名	方向	说明
1	Vref	Input	接口电平参考电压，通常可直接接电源
2	Vsupply	Input	电源
3	nTRST	Output	(可选项) JTAG复位。在目标端应加适当的上拉电阻以防止误触发。
4	GND	—	接地
5	TDI	Output	Test Data In from Dragon-ICE to target.
6	GND	—	接地
7	TMS	Output	Test Mode Select
8	GND	—	接地
9	TCK	Output	Test Clock output from Dragon-ICE to the target
10	GND	—	接地
11	RTCK	Input	(可选项) Return Test Clock。由目标端反馈给Dragon-ICE的时钟信号，用来同步TCK信号的产生。不使用时可以直接接地。
12	GND	—	接地
13	TDO	Input	Test Data Out from target to Dragon-ICE.
14	GND	—	接地
15	nSRST	Input/Output	(可选项) System Reset，与目标板上的系统复位信号相连。可以直接对目标系统复位，同时可以检测目标系统的复位情况。为了防止误触发，应在目标端加上适当的上拉电阻。
16	GND	—	接地
17	NC		保留
18	GND	—	接地
19	NC	—	保留
20	GND	—	接地

2．目标系统如何设计？

　

目标板使用与Dragon-ICE一样的20脚针座，信号排列见表1。RTCK和 nTRST这两个信号根据目标ASIC有否提供对应的引脚来选用。nSRST则根据目标系统的设计考虑来选择使用。下面是一个典型的连接关系图：

　　复位电路中可以根据不同的需要包含上电复位、手动复位等等功能。如果用户希望系统复位信号nSRST能同时触发JTAG口的复位信号nTRST，则可以使用一些简单的组合逻辑电路来达到要求。后面给出了一种电路方案的效果图。
　　

　图 3 一个复位电路结构的例子

　　在目标系统的PCB设计中，最好把JTAG接口放置得离目标ASIC近一些，如果这两者之间的连线过长，会影响JTAG口的通信速率。
　　另外电源的连线也需要加以额外考虑，因为Dragon-ICE要从目标板上吸取超过100mA的大电流。最好能有专门的敷铜层来供电，假如只能使用连线供电的话，最小线宽不应小于10mil (0.254mm)。

3. 14脚JTAG如何与20JTAG连接?

　 Dragon-ICE使用工业标准的20脚JTAG插头，但是有些老的系统采用一种14脚的插座

。这两类接口的信号排列如下：